U.S. patent number 5,368,617 [Application Number 07/876,496] was granted by the patent office on 1994-11-29 for process for reducing sulfur emissions with calcium-containing sorbents.
This patent grant is currently assigned to Genesis Research Corp.. Invention is credited to James K. Kindig.
United States Patent |
5,368,617 |
Kindig |
November 29, 1994 |
Process for reducing sulfur emissions with calcium-containing
sorbents
Abstract
An improved process for reducing sulfur oxide emissions from the
combustion of coal is disclosed wherein a fuel mixture comprising
calcium-containing sorbent and coal is fed to the burners and
sulfur oxides react with calcium from the sorbent in a high
temperature sulfur capture region, followed by additional sulfur
capture in a lower temperature, high humidity sulfur capture region
prior to separation of particulates from the flue gas. Sulfur
capture using calcium-containing sorbents can be combined with
aggressive coal beneficiation techniques to further enhance
reduction of sulfur oxide emissions. The process of the invention
provides a process for reducing sulfur oxides that efficiently uses
calcium-containing sorbents to enhance sulfur capture while
reducing the need for expensive equipment or process
modifications.
Inventors: |
Kindig; James K. (Boulder,
CO) |
Assignee: |
Genesis Research Corp.
(Carefree, AZ)
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Family
ID: |
22424731 |
Appl.
No.: |
07/876,496 |
Filed: |
April 30, 1992 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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775860 |
Oct 15, 1991 |
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492312 |
Mar 6, 1990 |
5096066 |
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126419 |
Nov 30, 1987 |
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Current U.S.
Class: |
44/622; 110/342;
110/343; 110/345; 423/235; 423/242.1; 423/244.07; 423/244.08;
44/580; 44/604 |
Current CPC
Class: |
B03B
5/30 (20130101); B03B 5/34 (20130101); B03B
5/442 (20130101); B03B 5/447 (20130101); B03B
9/005 (20130101); B03B 13/005 (20130101); C01G
49/06 (20130101); C01G 49/08 (20130101); C01P
2004/52 (20130101); C01P 2004/61 (20130101); C01P
2006/10 (20130101); Y02P 20/129 (20151101) |
Current International
Class: |
B03B
5/44 (20060101); B03B 13/00 (20060101); B03B
5/30 (20060101); B03B 5/34 (20060101); B03B
5/28 (20060101); B03B 9/00 (20060101); C01G
49/06 (20060101); C01G 49/08 (20060101); C01G
49/02 (20060101); C10L 009/00 (); C10L 010/00 ();
C10L 005/00 () |
Field of
Search: |
;423/235,244R,244A,242
;44/604,620,622,580 ;431/3 ;110/342,343,345 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Babcock & Wilcox, The Coal and Slurry Technology Associations
1992 Industry Handbook. .
Hall et al., Current Status of ADVACATE Process for Flue Gas
Desulfurization for Presentation at AWMA 1991 Annual Meeting,
Vancouver, B.C., Jun. 1991..
|
Primary Examiner: Medley; Margaret
Attorney, Agent or Firm: Sheridan Ross & McIntosh
Parent Case Text
REFERENCES TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. application Ser.
No. 07/775,860 filed on Oct. 15, 1991, which is a
continuation-in-part of U.S. application Ser. No. 07/492,312, now
U.S. Pat. No. 5,096,066, filed on Mar. 6, 1990, which is a
continuation of U.S. application Ser. No. 07/126,419 filed on Nov.
30, 1987, now abandoned.
Claims
I claim:
1. A process for reducing the emission of sulfur oxides from the
combustion of coal, comprising:
(a) combusting a fuel mixture of sulfur-containing coal and
calcium-containing sulfur sorbent, said coal comprising coal
particles resulting from aggressively beneficiating raw coal,
wherein said aggressive beneficiation comprises (i) separating the
raw coal into a coarse size fraction and a fine size fraction (ii)
density separating the coarse fraction into a clean coal fraction,
a refuse fraction and a middling fraction, (iii) comminuting said
middling fraction, and (iv) combining said comminuted middling
fraction with said fine fraction and further beneficiating the
combined fraction, said calcium-containing sulfur sorbent
comprising a calcium-containing compound selected from the group
consisting of lime, limestone, hydrated lime, calcium oxide,
dolomite, burnt dolomite, hydrated dolomite and combinations
thereof, at least a portion of calcium in said sulfur sorbent
reacts with sulfur oxides emitted during the combustion of coal,
said combusting producing a flue gas comprising sulfur and calcium
derived from said sulfur sorbent;
(b) following said combusting, allowing a first portion of said
calcium in the flue gas to react with a first portion of said
sulfur in the flue gas in a first stage of sulfur capture in a high
temperature region following combustion;
(c) following said reaction of said first portion of said calcium
in the flue gas, cooling said flue gas to a temperature within
about 100.degree. F. above the condensation temperature using heat
exchange or adding water to the flue gas said condensation
temperature is the temperature at which aqueous liquid condenses
from said flue gas; and
(d) following said cooling, allowing a second portion of said
calcium in the flue gas to react with a second mortion of said
sulfur in the flue gas at a temperature within about 100.degree. F.
above the condensation temperature of the flue gas stream in a
second stage of sulfur capture prior to particulate separation from
said flue gas, said second stage of sulfur capture having a
residence time of greater than about 0.1 second.
2. The process of claim 1, wherein said second stage of sulfur
capture comprises reacting said second portion of calcium with said
second portion of sulfur in the flue gas at a temperature within
50.degree. F. above the condensation temperature at the flue gas
stream.
3. The process of claim 1, wherein said second stage of sulfur
capture comprises reacting said second portion of calcium with said
second portion of sulfur in the flue gas at a temperature within
30.degree. F. above the condensation temperature at the flue gas
stream.
4. The process of claim 1, wherein said second stage of sulfur
capture comprises reacting said second portion of calcium with said
second portion of sulfur in the flue gas at a temperature within
15.degree. F. above the condensation temperature of the flue gas
stream.
5. The process of claim 1, wherein said second stage of sulfur
capture further comprises a residence time of greater than 1
second.
6. The process of claim 1, wherein said second stage of sulfur
capture further comprises a residence time of greater than 3
seconds.
7. The process of claim 1, further comprising separating
particulates from the flue gas stream after said second stage of
sulfur capture by electrostatic precipitation.
8. The process of claim 1, wherein said raw coal comprises pyrite
that is difficult to separate in that aggressive beneficiation of
said raw coal would result in rejecting less than 85% of the pyrite
in the raw coal while recovering about 85% of the BTU value in the
aggressively beneficiated coal relative to the raw coal.
9. The process of claim 3, wherein said raw coal comprises organic
sulfur in an amount greater than about 1.5% by weight on an
ash-free basis.
10. The process of claim 1, wherein said sulfur sorbent comprises a
pelletized intimate mixture of ultrafine coal particles and
limestone.
11. The process of claim 10, wherein said pelletized mixture
comprises coal particles of a size from about 0.015 mm to about
0.105 mm.
12. The process of claim 1, further comprising releasing sulfur
dioxide emissions to the atmosphere in an amount below about 1.2
pounds per million BTU of coal in the fuel mixture.
13. The process of claim 1, wherein said mixture of coal and sulfur
sorbent comprises ash-forming materials in an amount less than
about 20% by weight.
14. The process of claim 1, wherein said mixture of coal and sulfur
sorbent comprises ash-forming materials in an amount less than
about 15% by weight.
15. The process of claim 1, wherein said mixture of coal and sulfur
sorbent comprises ash-forming materials in an amount less than
about 12% by weight.
16. The process of claim 1, wherein said cooling is an air
preheater heat exchange after said first stage of sulfur capture
and before said second stage of sulfur capture.
17. The process of claim 1, wherein said calcium-containing sorbent
is selected from the group consisting of limestone, lime, dolomite,
and combinations thereof.
18. The process of claim 1, further comprising separating
particulates, including unreacted sulfur sorbent, from said flue
gas after said second stage of sulfur capture and injecting at
least a portion of said separated particulates into the flue gas
before said second stage of sulfur capture.
19. The process of claim 1, wherein said first stage of sulfur
capture further comprises reacting said first portion of sulfur
sorbent with said first portion of sulfur in the flue gas at a
temperature above 1500.degree. F.
Description
FIELD OF THE INVENTION
The present invention relates to an improved process for reducing
sulfur oxide emissions from combustion of coal using
calcium-containing sorbent.
BACKGROUND OF INVENTION
During the combustion of coal, and in particular combustion of many
lower grades of coal, sulfur oxides are produced which, if emitted
into the atmosphere can cause significant environmental pollution.
Several methods for reducing the amount of sulfur oxide emissions
that are released into the atmosphere have been developed.
One method is to burn low sulfur coal in an attempt to avoid
problems associated with excessive sulfur oxide emissions. However,
such fuel is not always readily available and the cost to transport
such high quality coal is in many cases prohibitive. Significant
deposits of high sulfur coal exist in the United States,
predominantly in the Eastern part of the nation. Therefore,
significant research has been conducted and several processes
developed for reducing emissions from such high sulfur coals.
It is known that, under certain conditions, calcium reacts with
sulfur oxides in flue gas from the combustion of sulfur-containing
coal to capture sulfur as calcium sulfite and/or calcium sulfate,
thereby reducing the release of sulfur oxide emissions.
It is also known that, calcium-containing sorbents can be added
directly to the combustion chamber to react with sulfur oxides in
the high temperature region following combustion. U.S. Pat. No.
4,824,441 by Kindig, issued Apr. 25, 1989 discuss several methods
that have been tried to improve the sulfur capturing capability of
calcium-containing sorbents in the high temperature region
following combustion. For example, various promoters and catalysts
can effect the efficiency of sulfur capture. Also, mixing the
sorbent material with the coal prior to combustion increases the
residence time of calcium with sulfur oxides in the high
temperature region in which calcium is reactive with sulfur
oxides.
It is also known that calcium will react with sulfur oxides in a
lower temperature environment, wherein the temperature is close to
the temperature at which water, and generally sulfuric acid
produced by water and sulfur oxides, condense from the flue gas
stream. Such condensation temperature is also sometimes referred to
as the saturation temperature or the adiabatic saturation
temperature.
U.S. Pat. No. 4,867,955 by Johnson, issued Sep. 19, 1989, discusses
the injection of calcium-containing sorbent and water into the flue
gas to promote reaction of calcium with sulfur oxides to effect
capture of sulfur in a lower temperature, humidified environment.
U.S. Pat. No. 4,867,955 also discusses injection of calcium
carbonate into the combustion chamber to effect calcination of the
calcium carbonate to calcium oxide. Such calcined sorbent is
thereafter reacted in a lower temperature, high humidity
environment to effect sulfur capture.
Some attempts have been made to combine sulfur capture in the high
temperature combustion region with sulfur capture in a subsequent
lower temperature, high humidity environment. U.S. Pat. No.
4,519,995 by Schrofelbauer et al., issued May 28, 1985 discusses a
process wherein lignite and calcium carbonate are pulverized and
fed together to coal dust burners. Some flue gas is recycled to the
combustion zone to reduce the temperature in that region. Prior to
entry of flue gas into the dust filter, the relative moisture of
the flue gas is increased by cooling the hot flue gas in a heat
exchanger and/or by injecting or spraying water into the hot flue
gas such that additional sulfur is captured in a lower temperature,
humidified environment by sorbent that has collected on the bag
filter surface. U.S. Pat. No. 5,002,743 by Kokkonen et al., issued
Mar. 26, 1991, discusses the addition of sorbent to the combustion
zone, independent of the fuel, followed by humidification to effect
reaction at a lower temperature.
As these references indicate, there is a need to efficiently reduce
environmental pollution by sulfur oxide emissions following
combustion of coal. More efficient and less costly sulfur removal
techniques are required to effectively use existing high sulfur
coal resources.
The process of the present invention involves capturing sulfur in a
high temperature region following combustion of coal and also in a
lower temperature, high humidity environment under conditions such
that the efficiency of sulfur capture is enhanced and the need for
expensive modifications of existing processes and equipment is
reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing one example of the effective limit of
sulfur capture in a high temperature sulfur capture region as a
function of ash-forming material in the coal.
FIG. 2 is a graph showing pyrite rejection vs. BTU recovery from a
Pittsburgh No. 8 Seam Coal using conventional and aggressive coal
washing techniques.
FIG. 3 is a graph showing sulfur content of clean coal product vs.
BTU recovery for a Pittsburgh No. 8 Seam Coal using conventional
and aggressive coal washing techniques.
SUMMARY OF THE INVENTION
The present invention involves a process for reducing sulfur oxide
emissions from coal fired facilities using calcium-containing
sorbents to capture sulfur in both a high temperature stage in the
vicinity of the combustion chamber and in a lower temperature, high
humidity stage downstream of the combustion chamber and upstream of
particulate separation equipment. Sorbent and coal are mixed and
fed together as a fuel mixture to the burners where the coal
combusts. In one embodiment, the fuel mixture containing sorbent
and coal is pelletized. In another embodiment, the coal particles
in the pellets comprise very fine particle coal.
In one embodiment of the invention, beneficiated coal, and
preferably aggressively beneficiated coal, is used in the sulfur
oxide reducing process. Beneficiation of coal is followed by
mixture with calcium-containing sorbents. In another embodiment,
high sulfur coal or coal from which pyrite is difficult to liberate
is the raw coal for the beneficiation process.
In another embodiment, flue gases, containing entrained ash and
unreacted sorbent, are humidified downstream of the air preheater,
but close enough to the air preheater to allow sufficient residence
time for the lower temperature, high humidity sulfur capture, prior
to entry of the flue gas into particulate separation equipment.
In another embodiment of the invention, unreacted sorbent can be
separated from the flue gas at various points and recycled for
injection back into the flue gas stream at or upstream of the point
of humidification for the low temperature sulfur capture
region.
DETAILED DESCRIPTION OF THE INVENTION
The present invention involves a process for reducing the release
of sulfur oxide emissions produced during the combustion of coal. A
fuel mixture containing coal calcium-containing sorbent particles
is fed to coal burners. A portion of sulfur oxides in the
combustion flue gases react with calcium provided by the sorbent to
capture sulfur in a high temperature region following combustion.
Calcium that has not reacted to capture sulfur in the high
temperature region exits the high temperature region with
combustion flue gases. At a temperature close to, but above, the
condensation temperature of water in the flue gas, some of this
unreacted calcium is then reacted with sulfur oxides remaining in
the flue gas to capture additional sulfur in a lower temperature
region before the flue gases enter particulate separation
equipment.
Suitable sorbents include any calcium-containing compound such as
limestone (CaCO.sub.3), lime (CaO) or dolomite
(CaCO.sub.3.MgCO.sub.3), and a more preferable sorbent is
limestone. If limestone is added to the fuel mixture, such
limestone is typically calcined to produce lime in the high
temperature region of the combustion zone. Dolomite will likewise
undergo calcination in the high temperature region. The fuel
mixture may contain promoters and/or catalyst, however such
additives are not required. For example, some suitable sorbents and
fuel mixtures are discussed in U.S. Pat. No. 4,824,441 by Kindig,
issued Apr. 25, 1989, the contents of which are incorporated herein
as if set forth in full.
In a preferred embodiment of the present invention, the fuel
mixture comprises a beneficiated coal, preferably an aggressively
beneficiated coal. As used herein, coal refers to all ranks of
anthracite, bituminous, sub-bituminous and lignitic materials. As
used herein, beneficiated coal refers to the product from cleaning
raw coal, also called run-of-mine coal, of some ash-forming
material, and preferably some ash-forming material containing
sulfur, prior to use of the coal as a combustion fuel. Aggressively
beneficiated coal, as used herein, refers to a product derived from
a coal cleaning process designed for efficient beneficiation of
fine particles, preferably particles smaller than about 0.5 mm.
Aggressively beneficiated coal includes a product derived from a
coal cleaning procedure comprising (1) recovering coarse clean
coal, (2) rejecting a coarse refuse, (3) comminuting the resulting
middlings to fines if required to assist liberation of pyrite
(typically minus 0.5 mm) and (4) efficiently beneficiating
comminuted fines and fines resulting from mining. Preferably, such
aggressive beneficiation is as discussed in copending application
Ser. No. 07/775,860 filed Oct. 15, 1992, entitled "Coal Cleaning
Process," the contents of which are incorporated herein as if set
forth in full. The purpose of beneficiating coal, or aggressively
beneficiating coal, is to remove pyritic sulfur, also sometimes
referred to as inorganic sulfur, from the raw coal and thereby
reducing sulfur to produce a clean coal product that contains less
sulfur and less ash-forming materials than the raw coal, thereby
reducing sulfur oxide emissions and ash production during
combustion.
Excessive ash formation during combustion creates significant
problems in the operation of coal fired facilities, such as
electrical power generation facilities. Complications caused by ash
formation are increased by the use of sorbent materials, whether
mixed with the coal in a fuel mixture or added separately to the
combustion chamber, because such sorbent materials form ash on
combustion. As used herein, ash refers to the combustion products
of the myriad inorganic minerals and constituents, regardless of
origin, associated with coal, including for example, the following
groups: shales, kaolin, sulfide, carbonate, chloride, oxide and
accessory minerals. Although the acceptable amount of ash varies,
the fuel mixture containing coal and sorbent should generally
contain less than about 20% of total ash-forming materials,
including ash-forming materials from the coal and the sorbent,
preferably less than 15% ash-forming materials, and more preferably
less than about 12% ash-forming materials.
The amount of sorbent that can be added to the combustion chamber
is effectively limited by the acceptable level of ash formation
during combustion, which acceptable level varies depending on the
specific facility design. Because beneficiated coal, and
particularly aggressively beneficated coal, contains a reduced
level of ash-forming materials relative to raw coal, a higher level
of sorbent can be added to beneficiated coal than raw coal, and an
even higher level of sorbent can be added to aggressively
beneficiated coal.
The process of the present invention is particularly useful when
insufficient sorbent can be mixed with coal in the fuel mixture to
effect the desired sulfur oxide emissions reduction in the high
temperature capture zone alone, without exceeding ash-forming
limitations. A relationship exists between the amount of sulfur
that can effectively be captured in the high temperature capture
zone and the level of ash in the coal from which a fuel mixture of
coal and sorbent is made. For example, FIG. 1 shows the amount of
sulfur allowable in coal with a given ash content, assuming that
ash-forming material in the fuel mixture is limited to 15% by
weight, that no more than 1.2 pounds of sulfur dioxide per million
BTU of coal is released into the atmosphere, and that sulfur is
captured only in a high temperature capture zone. The shaded zone
indicates conditions under which the process of the present
invention would be particularly useful since additional sulfur can
be captured in a lower temperature, high humidity region. As used
herein, pounds of sulfur dioxide per million BTU refers to the
theoretical amount of sulfur dioxide that would be produced if all
sulfur in the coal formed sulfur dioxide upon combustion relative
to the gross heating value of the coal.
In addition to reducing ash-forming materials, beneficiation of
coal, and particularly aggressive beneficiation, also reduces the
amount of pyritic sulfur in coal, thereby reducing the overall
level of sulfur capture required by sorbents subsequently mixed
with the coal. Pyritic sulfur, sometimes referred to as inorganic
sulfur, as used herein, refers to sulfur which is not chemically
bound in the coal matrix. Conversely, organic sulfur refers to that
sulfur chemically bound in the coal matrix.
According to the process of the present invention,
calcium-containing sorbents in the fuel mixture are reacted with
sulfur oxides to capture sulfur following combustion of the fuel
mixture in a high temperature region in the vicinity of the
combustion zone. Such high temperature sulfur capture using calcium
generally occurs at temperatures from about 2250.degree. F. to
about 1500.degree. F. Such calcium reacts with sulfur oxides to
form calcium sulfate, a solid which can be removed from the flue
gases in downstream particulate separation equipment. Using a fuel
mixture comprising intimately mixed coal and sorbent particles
promotes efficient sulfur capture in the high temperature region by
maximizing residence time in the high temperature zone, and by
providing maximum contact between sulfur oxides and calcium in the
sulfur sorbent. Additional discussion concerning high temperature
sulfur capture is presented in U.S. Pat. No. 4,824,441, supra.
Not all sorbent material however, reacts with sulfur oxides to
capture sulfur in the high temperature region. Calcium in the
sorbent reacts more slowly with sulfur oxides at temperatures below
about 1500.degree. F. It is known however that calcium in
calcium-containing sorbents reacts more quickly with sulfur oxides
at reduced temperature and increased humidity.
In a typical coal fired electrical power generation facility,
combustion flue gases exiting from the combustion zone are passed
through an air preheater, wherein heat in the flue gas is
transferred by heat exchange to incoming air to be used in the
combustion reaction. Following heat exchange in the air preheater,
the flue gases have generally been cooled to a temperature of from
about 275.degree. F. to about 375.degree. F. Additional sulfur
capture, using unreacted calcium, can be effected downstream of the
air preheater by increasing the humidity of the flue gas stream
such that the unreacted calcium becomes reactive with sulfur
oxides, thereby capturing additional sulfur in the form of calcium
sulfate or calcium sulfite.
In one embodiment of the present invention, following the air
preheater, flue gases are humidified such that the temperature of
the flue gases is within about 100.degree. F. above the
condensation temperature of such flue gas stream, preferably within
about 50.degree. F. above the condensation temperature, more
preferably within about 30.degree. F. above the condensation
temperature, and most preferably within about 15.degree. F. above
the condensation temperature. As used herein, condensation
temperature refers to that temperature at which water, and also
aqueous sulfuric acid created reaction of water with sulfur oxides,
condense from the flue gas. Such temperature is also referred to as
the saturation temperature and as the adiabatic saturation
temperature. Humidification can be accomplished either by heat
exchange, whereby the flue gas stream is cooled to a desired
temperature close to the condensation temperature, or by adding
water, preferably in the form of a fine spray, to the flue gas
stream, such that the added water is completely vaporized and the
corresponding humidity of the flue gas stream increases to the
desired level.
In a preferred embodiment, humidification occurs after the air
preheater, but as close to the air preheater as possible so as to
maximize the residence time for the lower temperature sulfur
capture reaction. Electrical power generation facilities generally
have particle separation equipment downstream of the air preheater
to remove particles from the flue gas stream prior to releasing
such flue gases to the environment. The flue gas process flow can
be modified to incorporate a special reactor space to provide
sufficient residence time for effective sulfur capture following
humidification. Preferably however, humidification of the flue gas
stream occurs sufficiently upstream of particulate separation such
that no additional reactor space or special reactor design is
required for such effective sulfur capture. Preferably, a residence
time for sulfur capture in this low temperature zone between the
point of humidification and the particle separation equipment is
greater than about 0.1 second, more preferably greater than about 1
second, and most preferably greater than about 3 seconds.
Performing the humidification step as close to the air preheater as
possible is particularly important when particulate separation is
by means, such as electrostatic precipitation or cycloning, wherein
no significant sulfur capture can be effected in the particulate
separation equipment.
In one embodiment of the invention, unreacted sorbent can be
separated from the flue gas stream, either before or after the low
temperature sulfur capture reaction, preferably after such low
temperature sulfur capture, and recycled and injected back into the
flue gas stream at or upstream of the point of humidification. Such
separation can be effected by typical particulate separation
equipment or other known separation techniques. Thus, the present
invention can be combined with a variety of known processes for
injecting sorbent into the flue gas stream upstream of the low
temperature sulfur capture region. Examples of such in-duct
injection processes are as disclosed in U.S. Pat. No. 4,867,955,
supra, U.S. Pat. No. 4,804,521 by Rochelle et al., issued Feb. 14,
1989; U.S. Pat. No. 4,931,264 by Rochelle et al., issued Jun. 5,
1990; U.S. Pat. No. 5,047,221 by Jozewicz et al., issued Sep. 10,
1991; and U.S. Pat. No. 5,047,222 by Rochelle et al., issued Sep.
10, 1991.
The present invention is particularly useful when a raw coal, prior
to beneficiation for use in the fuel mixture, contains a high
organic sulfur content and/or when a raw coal contains pyrite that
is difficult to liberate. Although such raw coals are often
subjected to aggressive beneficiation, it is generally not
practical or possible to beneficiate such raw coals to a clean coal
product that could be burned in the absence of sulfur sorbents to
effectively control sulfur oxide emissions. Although some sulfur
can be captured in a high temperature sulfur capture reaction as
discussed in U.S. Pat. No. 4,824,441, supra, sulfur removal by a
high temperature reaction alone is often not sufficient. Many of
these difficult coals can be effectively used by combining
aggressive coal beneficiaticn techniques, sulfur capture in a high
temperature region, and sulfur capture in a lower temperature, high
humidity region.
In one embodiment of the present invention, a raw coal, prior to
beneficiation for use in the fuel mixture with sorbent, contains a
high concentration of organic sulfur. A coal contains a high
organic sulfur content when such raw coal contains above about 1.5
percent organic sulfur by weight on an ash-free basis.
In another embodiment of the present invention, a raw coal, prior
to beneficiation for use in the fuel mixture with sorbent, and
regardless of sulfur content, contains pyrite that liberates with
difficulty. A coal contains pyrite that liberates with difficulty
if, even when using aggressive beneficiation techniques, less than
about 85 percent of the pyrite can be rejected while recovering
about 85 percent of the BTU content in the clean coal product
compared to the raw coal.
In one embodiment of the present invention, coal and sorbent are
intimately mixed and formed into an agglomerated pellet form. As
used herein, pellet includes all types of agglomerations, including
pellets, briquettes and extrusions. Such agglomeration assures that
the coal and sorbent will be intimately mixed during combustion,
thereby maximizing sulfur capture in the high temperature region.
Preferably, coal particles in the agglomeration are smaller than
about 0.5 mm. Additional information covering such agglomerations
is provided in application Ser. No. 07/775,860, supra.
The amount of sorbent added to the coal to form the fuel mixture
depends on the sulfur content of the coal and the amount of sulfur
reduction desired, and the acceptable level for ash-forming
materials. However, in the case of calcium-containing sorbents, the
sorbent is added in an amount preferably greater than about the
stoichiometric amount of calcium required assuming complete
reaction between calcium in the sorbent and sulfur in the coal,
more preferably greater than two times the stoichiometric amount,
and most preferably greater than about five times the
stoichiometric amount.
The following example is provided for the purpose of illustrating
the present invention and is not intended to limit the scope of the
invention.
EXAMPLE
This example demonstrates the application of the process of the
present invention to reduce sulfur emissions for a coal containing
a high concentration of organic sulfur and also containing pyrite
that liberates poorly. The example also demonstrates the control of
ash-forming materials in the coal and sorbent mixtures. The example
uses a Pittsburgh No. 8 coal from eastern Ohio, which contains
total sulfur equating to approximately 9.28 pounds of sulfur
dioxide emissions per million BTU if combusted in the raw state.
FIGS. 2 and 3 show the washability characteristics of this coal.
FIG. 2 shows the rate of pyrite rejection as a function of BTU
recovery in the clean coal relative to the raw coal. FIG. 3 shows
the sulfur content of the clean coal as a function of BTU recovery
both conventional beneficiation techniques and an aggressive
beneficiation technique.
For example, the raw coal, also called run-of-mine coal, is
subjected to aggressive beneficiation. The largest size coal
fraction (11/2.times.1/2 mm) is sink-floated at specific gravities
of 1.30 and 2.00. The 1.30 float is recovered as clean coal and the
2.00 sink is rejected. The middlings, 1.30 sink by 2.00 float, are
then added to the minus 1/2 mm material from the raw coal.
Ordinarily, aggressive beneficiation requires comminution of
middlings, however such comminution is not required for this
particular raw coal feed. The resulting mixture is then
sink-floated at several different specific gravities. The resulting
clean coal is then added to the 1.30 float for a composite clean
coal product.
Through this aggressive coal beneficiation process, the 9.28 pounds
of sulfur dioxide per million BTU for combusting raw coal is
reduced to approximately 6.03 pounds of sulfur dioxide per million
BTU in the beneficiated coal at a BTU recovery of approximately 86%
in the beneficiated coal relative to the raw coal.
Limestone is then mixed with the ultra-fine particles of the
beneficiated coal and the mixture is pelletized. As used in this
example, ultra-fine coal particles refers to those coal particles
from about 0.015 mm to about 0.105 mm in size. The pellets are then
combined with the remaining coal particles. The amount of limestone
added is 14.5% based on the total weight of the overall coal and
sorbent mixture. The calcium-to-sulfur stoichiometry in the
resulting product is 1.25.
An estimated 14% of the sulfur in this fuel mixture is captured in
the boiler through sulfation of the sorbent in the high temperature
sulfur capture region. The emission level from the boiler is
therefore approximately 5.20 pounds of sulfur dioxide per million
BTU.
The flue gas existing the boiler contains unreacted sulfur dioxide
and entrained ash, including unreacted calcium in the form of lime.
The molar ratio of unreacted calcium to sulfur dioxide exiting the
boiler is approximately 1.3. Water is then added to the flue gas to
adjust the temperature to just above the condensation temperature
to effect additional sulfur capture. Following this low temperature
sulfur capture, sulfur dioxide emissions are reduced to 1.2 pounds
per million BTU, in compliance with the year 2000 standard of the
United States Clean Air Act Amendments of 1990. Results of the
example are summarized in Table I.
Table I shows the stepwise reduction in sulfur emissions. Table I
also shows that, by using aggressively beneficiated coal, sorbent
can be added to the coal without causing excessive ash production,
thus allowing the addition of more sorbent than if the coal were
not aggressively beneficiated.
TABLE I ______________________________________ Ash-Forming Lbs.
SO.sub.2 mtl., % per million by weight BTU
______________________________________ Raw Coal 26.9 9.28
Beneficiated Coal 7.6 6.03 Coal/Sorbent Fuel Mixture 16.0 6.03
After High Temperature n/a 5.20 Sulfur Capture After Low
Temperature n/a 1.20 Sulfur Capture
______________________________________
While various embodiments of the present invention have been
described in detail, it is apparent that modifications and
adaptations of those embodiments will occur to those skilled in the
art. However, it is to be expressly understood that such
modifications and adaptations are within the scope of the present
invention, as set forth in the following claims.
* * * * *